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Chapter 5 Arrays Introducing Arrays Declaring Array Variables, Creating Arrays, and Initializing Arrays Passing Arrays to Methods Copying Arrays Multidimensional Arrays Search and Sorting Methods Introducing Arrays Array is a data structure that represents a collection of the same types of data. double[] myList = new double[10]; myList reference myList[0] myList[1] myList[2] myList[3] myList[4] myList[5] myList[6] myList[7] myList[8] myList[9] An Array of 10 Elements of type double Declaring Array Variables datatype[] arrayname; Example: double[] myList; datatype arrayname[]; Example: double myList[]; Creating Arrays arrayName = new datatype[arraySize]; Example: myList = new double[10]; references the first element in the array. myList[9] references the last element in the array. myList[0] Declaring and Creating in One Step datatype[] arrayname = new datatype[arraySize]; double[] myList = new double[10]; datatype arrayname[] = new datatype[arraySize]; double myList[] = new double[10]; The Length of Arrays Once an array is created, its size is fixed. It cannot be changed. You can find its size using arrayVariable.length For example, myList.length returns 10 Initializing Arrays Using a loop: for (int i = 0; i < myList.length; i++) myList[i] = i; Declaring, creating, initializing in one step: double[] myList = {1.9, 2.9, 3.4, 3.5}; This shorthand syntax must be in one statement. Declaring, creating, initializing Using the Shorthand Notation double[] myList = {1.9, 2.9, 3.4, 3.5}; This shorthand notation is equivalent to the following statements: double[] myList = new double[4]; myList[0] = 1.9; myList[1] = 2.9; myList[2] = 3.4; myList[3] = 3.5; CAUTION Using the shorthand notation, you have to declare, create, and initialize the array all in one statement. Splitting it would cause a syntax error. For example, the following is wrong: double[] myList; myList = {1.9, 2.9, 3.4, 3.5}; Example 5.1 Testing Arrays Objective: The program receives 6 numbers from the keyboard, finds the largest number and counts the occurrence of the largest number entered from the keyboard. Suppose you entered 3, 5, 2, 5, 5, and 5, the largest number is 5 and its occurrence count is 4. TestArray Run Example 5.2 Assigning Grades Objective: read student scores (int) from the keyboard, get the best score, and then assign grades based on the following scheme: – Grade is A if score is >= best–10; – Grade is B if score is >= best–20; – Grade is C if score is >= best–30; – Grade is D if score is >= best–40; – Grade is F otherwise. AssignGrade Run Passing Arrays to Methods Java uses pass by value to pass parameters to a method. There are important differences between passing a value of variables of primitive data types and passing arrays. For a parameter of a primitive type value, the actual value is passed. Changing the value of the local parameter inside the method does not affect the value of the variable outside the method. For a parameter of an array type, the value of the parameter contains a Example 5.3 Passing Arrays as Arguments Objective: Demonstrate differences of passing primitive data type variables and array variables. TestPassArray Run Example 5.3, cont. swap(a[0], a[1]) a[0] 1 a[1] 2 n1 1 n2 2 Pass by value swap( n1, n2) swapFirstTwoInArray(a) a Reference : Pass by value (Reference value) swapFirstTwoInArray(array) array Reference a[0] a[1] Example 5.4 Computing Deviation Using Arrays n m ean xi i 1 n Deviation n deviation 2 ( x m ean ) i i 1 n 1 Run Example 5.5 Counting Occurrence of Each Letter Generate 100 lowercase letters randomly and assign to an array of characters. Count the occurrence of each letter in the array. Find the mean and standard deviation of the counts. CountLettersInArray Run Example 5.6 Copying Arrays In this example, you will see that a simple assignment cannot copy arrays in the following program. The program simply creates two arrays and attempts to copy one to the other, using an assignment statement. TestCopyArray Run Copying Arrays Before the assignment list2 = list1; list1 After the assignment list2 = list1; Contents of list1 list2 list1 Contents of list1 list2 Contents of list2 Garbage Contents of list2 Copying Arrays Using a loop: int[] sourceArray = {2, 3, 1, 5, 10}; int[] targetArray = new int[sourceArray.length]; for (int i = 0; i < sourceArrays.length; i++) targetArray[i] = sourceArray[i]; The arraycopy Utility arraycopy(sourceArray, src_pos, targetArray, tar_pos, length); Example: System.arraycopy(sourceArray, 0, targetArray, 0, sourceArray.length); Multidimensional Arrays Declaring Variables of Multidimensional Arrays and Creating Multidimensional Arrays int[][] matrix = new int[10][10]; or int matrix[][] = new int[10][10]; matrix[0][0] = 3; for (int i=0; i<matrix.length; i++) for (int j=0; j<matrix[i].length; j++) { matrix[i][j] = (int)(Math.random()*1000); } double[][] x; Multidimensional Array Illustration 0 1 2 3 4 0 1 2 3 4 0 0 0 0 1 1 1 2 2 2 3 3 4 4 matrix = new int[5][5]; 7 matrix[2][1] = 7; 3 1 1 2 2 3 4 5 6 7 8 9 10 11 12 int[][] array = { {1, 2, 3}, {4, 5, 6}, {7, 8, 9}, {10, 11, 12} }; Declaring, Creating, and Initializing Using Shorthand Notations You can also use a shorthand notation to declare, create and initialize a two-dimensional array. For example, int[][] array = { {1, 2, 3}, {4, 5, 6}, {7, 8, 9}, {10, 11, 12} }; This is equivalent to the following statements: int[][] array array[0][0] = array[1][0] = array[2][0] = array[3][0] = = new int[4][3]; 1; array[0][1] = 2; array[0][2] = 4; array[1][1] = 5; array[1][2] = 7; array[2][1] = 8; array[2][2] = 10; array[3][1] = 11; array[3][2] 3; 6; 9; = 12; Lengths of Multidimensional Arrays int[][] array = { {1, 2, 3}, {4, 5, 6}, {7, 8, 9}, {10, 11, 12} }; array.length array[0].length array[1].length array[2].length Ragged Arrays Each row in a two-dimensional array is itself an array. So, the rows can have different lengths. Such an array is known as a ragged array. For example, int[][] matrix = { {1, 2, 3, 4, 5}, {2, 3, 4, 5}, {3, 4, 5}, {4, 5}, {5} }; Example 5.7 Adding and Multiplying Two Matrices Objective: Use two-dimensional arrays to create two matrices, and then add and multiply the two matrices. a11 a12 a13 a14 a15 b11 b12 b13 b14 b15 a11 b11 a 21 a 22 a 23 a 24 a 25 b 21 b 22 b 23 b 24 b 25 a 21 b21 a 31 a 32 a 33 a 34 a 35 b31 b32 b33 b34 b35 a 31 b31 a 41 a 42 a 43 a 44 a 45 b41 b42 b43 b44 b45 a 41 b41 a 51 a 52 a 53 a 54 a 55 b51 b52 b53 b54 b55 a 51 b51 a12 b12 a13 b13 a14 b14 a15 b15 a 22 b22 a 23 b23 a 24 b24 a 25 b25 a 32 b32 a 33 b33 a 34 b34 a 35 b35 a 42 b42 a 43 b43 a 44 b44 a 45 b45 a 52 b52 a 53 b53 a 54 b54 a 55 b55 TestMatrixOperation Run Example 5.7 (cont) Adding and Multiplying Two Matrices a11 a12 a13 a14 a15 b11 b12 b13 b14 b15 c11 c12 c13 c14 c15 a 21 a 22 a 23 a 24 a 25 b21 b22 b23 b24 b25 c 21 c 22 c 23 c 24 c 25 a 31 a 32 a 33 a 34 a 35 b31 b32 b33 b34 b35 c 31 c 32 c 33 c 34 c 35 a 41 a 42 a 43 a 44 a 45 b41 b42 b43 b44 b45 c 41 c 42 c 43 c 44 c 45 a 51 a 52 a 53 a 54 a 55 b51 b52 b53 b54 b55 c 51 c 52 c 53 c 54 c 55 cij = ai1b1j+ai2b2j+ai3b3j+ai4b4j+ai5b5j Example 5.8 Grading Multiple-Choice Test Students’ Answers to the Questions: 0 1 2 3 4 5 6 7 8 9 Student Student Student Student Student Student Student Student 0 1 2 3 4 5 6 7 A D E C A B B E B B D B B B B B A A D A D E A E C B A E C C C C C C C D C C C C D A B C D D D D E E E E E E E E E E E E E E E E A A A A A A A A D D D D D D D D Objective: write a program that grades multiple-choice test. Key to the Questions: 0 1 2 3 4 5 6 7 8 9 Key D B D C C D A E A D Grade Exam Run Example 5.9 Calculating Total Scores Objective: write a program that calculates the total score for students in a class. Suppose the scores are stored in a threedimensional array named scores. The first index in scores refers to a student, the second refers to an exam, and the third refers to the part of the exam. Suppose there are 7 students, 5 exams, and each exam has two parts--the multiple-choice part and the programming part. So, scores[i][j][0] represents the score on the multiple-choice part for the i’s student on the j’s exam. Your program displays the total score for each student, . TotalScore Run Searching Arrays Searching is the process of looking for a specific element in an array; for example, discovering whether a certain score is included in a list of scores. Searching, like sorting, is a common task in computer programming. There are many algorithms and data structures devoted to searching. In this section, two Linear Search The linear search approach compares the key element, key, with each element in the array list[]. The method continues to do so until the key matches an element in the list or the list is exhausted without a match being found. If a match is made, the linear search returns the index of the element in the array that matches the key. If Example 5.10 Testing Linear Search Objective: Implement and test the linear search method by creating an array of 10 elements of int type randomly and then display this array. Prompt the user to enter a key for testing the linear search. LinearSearch Run Binary Search For binary search to work, the elements in the array must already be ordered. Without loss of generality, assume that the array is in ascending order. e.g. 2 4 7 10 11 45 50 59 60 66 69 70 79 The binary search first compares the key with the Binary Search, cont. If the key is less than the middle element, you only need to search the key in the first half of the array. If the key is equal to the middle element, the search ends with a match. If the key is greater than the middle element, you only need to search the key in the second Binary Search, cont. key = 11 [0] [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] list 2 4 7 10 11 45 key < 50 mid [0] [1] [2] [3] [4] [5] 2 key > 7 4 7 10 11 45 mid [3] [4] [5] 10 11 45 key = 11 50 59 60 66 69 70 79 mid Example 5.11 Testing Binary Search Implement and test the binary search method. The program first creates an array of 10 elements of int type. It displays this array and then prompts the user to enter a key for testing binary search. Objective: BinarySearch Run Example 5.12 Using Arrays in Sorting Objective: Use the selectionSort method to write a program that will sort a list of double floating-point numbers. int[] myList = {2, 9, 5, 4, 8, 1, 6}; // Unsorted Sort it to produce 1, 2, 4, 5, 6, 8, 9 2, 9, 5, 4, 8, 1, 6 SelectionSort Run Example 5.12: (Cont) Using Arrays in Sorting int[] myList = {2, 9, 5, 4, 8, 1, 6}; // Unsorted Find the largest element in myList and swap it with the last element in myList. 2, 9, 5, 4, 8, 1, 6 => 2, 6, 5, 4, 8, 1, 9 (size = 7) 2, 6, 5, 4, 8, 1 => 2, 6, 5, 4, 1, 8 (size = 6) 2, 6, 5, 4, 1 => 2, 1, 5, 4, 6 (size = 5) 2, 1, 5, 4 => 2, 1, 4, 5 2, 1, 4 => 2, 1, 4, 2, 1 => 1, 2 Sort it to produce 1, 2, 4, 5, 6, 8, 9 Exercise 5.5: Bubble Sort int[] myList = {2, 9, 5, 4, 8, 1, 6}; // Unsorted Pass 1: Pass 2: Pass 3: Pass 4: Pass 5: Pass 6: 2, 5, 4, 8, 1, 6, 9 2, 4, 5, 1, 6, 8, 9 2, 4, 1, 5, 6, 8, 9 2, 1, 4, 5, 6, 8, 9 1, 2, 4, 5, 6, 8, 9 1, 2, 4, 5, 6, 8, 9